Rotary impact motor

ABSTRACT

In a rotary impact motor for power driven fastener setting tools, an impact dog is pivotally journalled on a rotatable anvil about an axis spaced from the axis of rotation of the anvil. A hammer is rotatable around the impact dog and anvil. Cooperating cam means mounted respectively on the hammer and on the impact dog are responsive to relative rotation therebetween for pivoting out the impact dog from the anvil to be impacted by the hammer.

United States Patent [191 Schoeps et al.

[4 1 Feb. 5, 1974 ROTARY IMPACT MOTOR 3,648,784 3/1972 Schoeps 173/915 Inventors: Knut Christian p Karl G" 3,179,219 4/1965 Karden l7 3/93.5

Kiirdn, both of Nacka, Sweden Primary Examiner-Stephen J. Novosad [73] Asslgnee' Q Capco Akuebolag Nacka Assistant ExaminerWilliam F. Pate, III

we en Attorney, Agent, or Firm-Flynn & Frishauf [22] Filed: Mar. 31, 1972 [21] App]. No.: 239,881 [57] ABSTRACT [30] Foreign Application Priority Data In a rotary impact motor for power driven fastener A 7 1971 S d 4515 71 setting tools, an impact dog is pivotally joumalled on a we en rotatable anvil about an axis spaced from the axis of rotation of the anvil. A hammer is rotatable around C(i1. the impact dog and anviL Cooperating cam means [58] Fie'ld 6 93 7 94 mounted respectively on the hammer and on the impact dog are responsive to relative rotation therebetween for pivoting out the impact dog from the anvil [56] g fizg g gzf Ts to be impacted by the hammer.

3,610,344 10/1971 Schoeps et a1 173/935 14 Claims, 12 Drawing Figures ROTARY IMPACT MOTOR This invention relates to improvements in power operated impact tools such as wrenches for tigthening or loosening nuts or bolts by power, and more particularly to a rotary impact motor utilised in such impact tools for producing rotary blows acting on the anvil member.

In such applications is formerly known a type of rotary impact motor provided with a rotatable anvil, a hammer coaxially rotatably carried with respect to the anvil, and an impact dog guided on the anvil for compulsory rotation together therewith. The impact dog is movably arranged relative to the hammer and the anvil for taking respectively an impact position and a release position therebetween under the influence respectively of cam means adjoining the impact dog and of a power bias acting thereon. In hitherto suggested rotary impact motors of the above kind radial or axial movement has been utilized for purposes of displacing the impact dog relative to the anvil. In constructions incorporating a radially movable impact dog, the guide channels for the impact body in the anvil cause a non-desirable but unavoidable weakening of the anvil. In constructions with an axially movable impact dog the presence of an axial release position increases the length of the impact motor which is a fact having an unfavourable effect on handling of hand tools incorporating such an impact motor.

It is therefore an object of the invention, to provide an impact motor of the type in question, wherein a third movement alternative is relied upon for the impact body. As a result, the impact motor can be given an axially more compact and easily handled embodiment without any critical weakening of the anvil.

The above and other objects of the invention will become obvious from the following description and from the accompanying drawings in which an embodiment of the invention is illustrated by way of example. It should be understood that this embodiment is only illustrative of the invention and that various modifications may be made within the scope of the claims without departing from the scope of the invention.

In the drawings FIG. 1 is a longitudinal section through an impact motor according to the invention.

FIG. 2 is a cross section on the line 2-2 and FIG. 3 a cross section on the line 33 in FIG. 1 with the impact motor in impact position.

FIG. 4 is a section corresponding to FIG. 3 but with the impact motor in release position after impact.

FIGS. 5 7 show diagrammatically the hammer, the impact body, and the cam body of the impact motor respectively during, after and before the delivery of an impact, the hammer, which is annular in cross section, being cut up for purposes of illustration and shown in a plane development.

FIGS. 8 10 are perspective views of respectively an impact dog, a cam body cooperating therewith, and a pin uniting said parts when they are in assembled position thereon.

FIG. 11 is a section corresponding to FIG. 3 but with the impact motor in release position just before the delivery of an impact.

FIG. 12, finally, is a view taken on the line l2l2 in FIG. 1.

In the Figures the impact clutch is fitted in a hand held impact wrench 15. The wrench 15 comprises a housing 16 having a reversible motor, not shown in the Figures, for example a pneumatically driven one. The motor effects rotation of a drive shaft 17 journalled in the housing 16 selectively in the desired rotational direction. The rotary impact motor proper is mounted between thehousing l6 and afront portion 18 which in a journal bore 19 rotatably supports a forwardly projecting anvil 20. Theforward end of the anvil 20 is a polygonal driving end 21 intended for removably carrying a tool thereon, not shown, usually a socket wrench.

The drive shaft 17 is provided with splines 22 which mate with inner splines 23 at the rear end of a hammer 24. With a cylindrical bore 25 the hammer 24 surrounds in tubular way the rear end of the anvil 20 and is rotatably supported thereon. The hammer 24 is provided with two peripherally spaced or diametrically disposed recesses 26, 27, FIG. 2, opening into the bore 25, and each having a partly cylindrical impact surface 28 and 29, respectively facing in opposite rotational directions of the hammer 24. Inside the hammer 24 behind the bore 25 there is arranged an annular groove 30 reaching to the depth of the recesses 26, 27 and bordered at the rear thereof by an end wall 31 within the hammer 24. The end wall 31 extends from a hub 32 incorporating the splines 23. At the periphery of the hammer 24 the end wall 31 has a peripheral recess and through slot 33 therein, FIG. 12, forming an arc of a circle. The recess and slot 33 opens into the annular groove 30 and receives a pivot 34 which at the forward end thereof carries a cam roller 35 received in the annular groove 30. The hub 32 carries pivotally thereon a holder plate 36 which at a forwardly bent radial shank carries the pivot 34 and by the base portion thereof on the one hand is pivotally carried by the hub 31 and on the other bears against a bearing arranged for the drive shaft 17 in the housing 16.

The anvil 20 is provided with a central circular shoulder 38 which is rotatably supported by a cylindrical inner bearing surface 39 in the front portion 18. Behind the shoulder 38 the anvil 20 has a thickened portion 40 which at opposed sides of a peripheral arcuate groove 37, FIG. 2, provides journal ribs for the bore 25 of the hammer 24. Behind the portion 40 the anvil abuts rotatably against the end wall 31 by the end face of a reduced cylindrical end portion 42, FIG. 3. An axial, partly cylindrical recess 43 extends along the anvil 20 through the shoulder 38 and the portions 40, 42. The radius of curvature of the recess 43 is equal to that of the impact surfaces 28, 29. Within the anvil 20 a double-bent torsion spring 45 is inserted in a guiding passage 44 axially obliquely orientated relative to the anvil. The torsion spring 45 is clamped by two angularly bent and laterally protruding shank ends 46, 47 against opposed abutments on a rib 48 disposed in the middle plane of the recess 43 on the end face of the end portion 42. The shank ends 46, 47 project in a radial direction past the rib 48 across the main portion of the partly cylindrical recess 43, FIG. 4. The shank ends 46, 47 are free to pivot in grooves 49, 50 in the end face of portion 42 at both sides of the rib 48. The shank ends 46, 47 are retained in the grooves 49, 50 by the end wall 31 of the hammer 24.

By a partly cylindrical back 52 an impact dog 51, FIG. 8, is supported and pivotally journalled in the recess 43, the impact dog having an overturned V-shaped guiding shoulder 53 at its forward end and a narrow radial guiding flange 54 at its rear end, both being symmetrical relative to a central plane of symmetry of the 1 impact dog 51. In this plane of symmetry the impact dog has a longitudinal axial bore 60 for a pin 55, FIG. 10. A flattened or generally ovoid cam 56 is mounted between a head on the pin 55 and the guiding flange 54 of the impact dog 51, FIG. 9. The cam 56 is pivotally journalled on the pin 55 at a bore 61. By means of a central spacing or return lug 57 and the head of the pin 55, the cam 56 is kept pivotally in place in the recess 43 between the guiding flange 54 and the end wall 31. The cam 56 has a pointed cam crest 58 disposed symmetrically in a central plane together with the bore 61 and the spacing lug 57 and is intended for cooperation with the cam roller 35 carried by the hammer 24. The portion of the cam 56 turned away from the cam crest 58 is symmetric with respect to the central plane of the latter. The opposite flattened sides 63, 64 of the ovoid cam 56 are partly cylindrical with the same radius of curvature as the recess 43. The head of the pin 55 and the spacing lug 57 are clamped between or straddled by the shank ends 46, 47 of the torsion spring 45 and are maintained by them in radial alignment with the rib 48 owing to the pre-compression of the torsion spring 45. Thus the impact body 51 and the ovoid cam 56 are biased by the torsion spring 45 to take a central symmetrical position in the anvil 20, FIGS. 4, 11. The radially directed outer face 63 of the impact dog 51 turned away from the part cylinder surface 52 has a radius of curvature substantially equal to the one of the anvil 21 at the cylindrical portion 40 and is concentric with the axis of rotation thereof.

In operation of the impact wrench let it be supposed that the drive shaft 17 rotates the impact motor clockwise when the details thereof in FIG. 1 are viewed in the direction of the arrows 2, 3. Via the impact motor the drive shaft 17 rotates the driving end 21 of the anvil which transmits rotation to a socket wrench and a screw, not shown. As long as the screw rotates easily, the parts of the rotary impact motor due to friction will stay in the prevailing position, for example in the position immediately before an impact is delivered as shown in FIG. 11. The drive shaft 17 rotates the hammer 24 via the drive connection splines 22, 23 and the rotation is transmitted to the anvil 20 via the end of the slot 33 trailing with respect to the direction of rotation, FIG. 12, and further via the cam lug 34, the cam roller 35, the ovoid cam 56, the shank ends 46, 47 of the torsion spring 45, and the rib 48. The rotation goes on continually until the screw, upon having been screwed down and due to increased friction, offers a sufficiently high resistance to rotation.

At increasing rotational resistance the anvil 20 stops while the directly driven hammer 24 goes on rotating. The cam roller 35 together with the hammer 24 is displaced angularly from the position in FIG. 1 1 in the forward direction to the position in FIGS. 3, 5. During this movement the cam roller 35 entrains the crest 58 of the cam 56 and pivots the ovoid cam 56 down in the clockwise direction against the recess 43. With the side surface 64 of the cam 56 depressed against the recess 43 the cam 56 slides backwards therealong, at which instant the head of the pin 55 and the spacing lug 57 brace apart the shank ends 46, 47 of the torsion spring 45 and the rearwardly moving pin 55 simultaneously therewith pivots the peripheral end of the impact dog 51 remote from the cam roller 35 upwards in the clockwise direction into the recess of the hammer 24 and into the path of movement of the impact surface 28 thereof. The impact surface 28 which faces the rotational direction of the hammer 24 hits the impact dog 51, FIGS. 2, 5. As a result the hammer 24 is momentarily arrested and delivers its kinetic energy in the form of a rotational impact to the impact dog 51 occupying the impact position. The impact is transmitted via the recess 43 to the anvil 20. When the hammer 24 is stopped dead, the cam roller 35, due to the inertia thereof and of the holder 36, continues the rotary movement for a certain distance in the slot 33, FIGS. 4, 6, until friction stops the holder 36. As the cam roller 35 thus has left the crest 58, the braced-apart shank ends 46, 47 of the torsion spring 45 can return against the opposed abutment of the rib 48 and the ovoid cam 56 is pivoted up and raised to its symmetrical position together with the impact dog 51, FIGS. 4, 5. The impact dog 51 takes up a release position within the path of rotation of the impact surfaces 28, 29 and with the outer face 63 of the impact dog 51 lying flush with the periphery of the anvil middle portion 40. With the impact dog 51 thus out of the recess 26 and out of the way of the impact surface 28, the hammer is free and starts moving accelerated angularly by the drive shaft 17. This movement continues until the cam roller 35 again hits the crest 58 of the ovoid cam 56, FIG. 7, whereby at the first instant the cam roller 35 is brought to the end of the slot 33 trailing with respect to the rotational direction, FIG. 12, whereafter the cam roller 35 is positively entrained by that end of the slot and turns down the ovoid cam 56 in clockwise direction pivoting the impact dog 51 in the same sense of rotation into impact position, FIG. 2. A new powerful blow is now delivered as a result of the free acceleration of the hammer 24. The impact cycle is thereafter repeated in the above described manner blow after blow until the desired torque has been obtained in the screw.

When the rotational direction is reversed for rotation in the counter-clockwise direction, the cam roller 35, for instance in FIG. 6, will hit the cam crest 58 from the other side and the roller 35 is at first brought to the end of the slot 33 which now trails with respect to the new rotational direction. The cam roller 35 thereafter depresses the cam crest 58 in counter-clockwise direction thereby pivoting the impact dog 51 counter-clockwise to the impact position. The end of the impact dog 51 turned away from the cam crest 58 is now swung out into the recess 27 and an impact is delivered thereagainst by means of the impact surface 29 of the hammer 24. As a result of the symmetrical design of the impact motor and of the length of the groove 33 having been chosen in the correct relation to the peripheral distance of the impact surfaces 28, 29, impact delivery is achieved irrespective of the sense of rotation.

The impact dog 51 is held in position in the recess 43 due to the fact that the guiding shoulder 53 by its tip abuts against the bearing surface 39 on the front portion 18 while the guiding flange 54 of the impact dog 51 by its tip in a similar way abuts against the annular groove of the hammer 24. The guiding flange 54' is disposed axially outside the path of movementof the cam roller and does not hinder the cooperating thereof with the cam crest 58. The impact surfaces 28, 29 of the hammer body 24 have the same radius of curvature as the back portion 52 of the impact dog 51 against the opposed peripheral ends of which they deliver their impacts.

What we claim is:

1. A rotary impact motor comprising a rotatable anvil, a hammer rotatably supported in coaxial relation with respect to said anvil, means for rotating said hammer, impact surface means on a portion of said hammer rotatable around said anvil, an impact dog pivotally journalled on said anvil radially inside the path of movement of said impact surface means and about an axis spaced from the axis of rotation of said anvil between an inswung release position within the path of rotation of said impact surface means and an outswung impact position intersecting said path of rotation, biasing means between said anvil and said impact dog for swinging in said impact dog to said release position thereof, and cam means between said hammer and said impact dog responsive to relative rotation therebetween for swinging out said impact dog from said release to said impact position into said path of rotation of said impact surface means.

2. A rotary impact motor according to claim 1 in which said cam means comprise a first cam means on said impact dog and cooperating second cam means mounted on said hammer for swinging out said impact dog to said impact position in same sense of swinging as the sense of rotation of said hammer.

3. A rotary impact motor according to claim 1 in which said biasing means is a torsion spring extending axially of said impact motor, said torsion spring being non-rotatably associated with said anvil for swinging in said impact dog to said release position thereof.

4. A rotary impact motor according to claim 3 in which said torsion spring is double shanked and provided with laterally protruding shank ends, opposed abutments on said anvil, a pin supported by said impact dog, and said opposed abutments and pin being resiliently straddled by said shank ends.

5. A rotary impact motor according to claim 4, in which said cam means comprise a first cam means pivotally mounted on said pin of said impact dog and cooperating second cam means mounted on said hammer for swinging out said impact dog to said impact position thereof, a lug on said first cam means radially outwardly of said pin, and said lug being resiliently straddled by said shank ends.

6. A rotary impact motor according to claim 1 in which said impact dog is elongated and has a partly cylindrical back, a partly cylindrical axial recess on said anvil member, and said recess being equiform with said back and slidably receiving said impact dog therein for pivotal movement relative to said anvil.

7. A rotary impact motor according to claim 6 in which a radially directed face is provided on said impact dog opposite to said back thereof and concentric with the axis of rotation of said anvil when said impact dog is in the release position thereof.

8. A rotary impact motor comprising a rotatable anvil, a hammer rotatably supported in coaxial relation with respect to said anvil, means for rotating said hammer bidirectionally, a pair of impact surfaces on peripherally spaced portions of said hammer facing oppositely in peripheral direction thereof and rotatable around said anvil, an elongated impact dog pivotally journalled on said anvil about an axis spaced from the axis of rotation of said anvil, said impact dog being pivoted between a release position with the opposed peripheral ends thereof within the path of rotation of said impact surfaces and an impact position with the impact dog pivoted in the same sense as the chosen sense of rotation of said hammer for swinging out one of said ends to intersect said path of rotation, biasing means between said anvil and said impact dog for returning said impact dog to said release position thereof, a first cam means on said impact dog, a second cam means mounted on said hammer and cooperating with said first cam means for pivoting said impact dog to the impact position thereof in response to relative rotation between said hammer and said impact dog, and means for mounting said second cam means on said hammer with a peripheral play corresponding to the peripheral spacing of said impact surfaces for causing said outswung end to be impacted by the one of said impact surfaces facing the chosen rotational direction.

9. A rotary impact motor according to claim 8 in which said biasing means is a double shanked torsion spring provided with laterally protruding shank ends, opposed abutments on said anvil, a pin supported by said impact dog, and said opposed abutments and pin being resiliently straddled by said shank ends.

10. A rotary impact motor according to claim 9, in which said cam means comprise a first cam pivotally mounted on said pin of said impact dog and a cooperating second cam mounted on said hammer for swinging out said impact dog to said impact position thereof, a lug on said first cam radially outwardly of said pin, and said lug being resiliently straddled by said shank ends.

11. A rotary impact motor according to claim 1 in which said impact dog is symmetrical with respect to a central plane therethrough, said central plane in the release position of said impact dog substantially coinciding with a plane through the axis of rotation of the anvil and the pivoting axis of said impact dog.

12. A rotary impact motor according to claim 8 in which said impact dog is symmetrical with respect to a central plane therethrough, said central plane in the release position of said impact dog substantially coinciding with a plane through the axis of rotation of said anvil and the pivoting axis of said impact body.

13. A rotary impact motor comprising a rotatable anvil, a tubular hammer supported in coaxial relation rotatably around said anvil, means for rotating said hammer, an elongated impact dog having a partly cylindrical back formed thereon, a partly cylindrical axial recess on said anvil member, said recess being equiform with said back and slidably receiving said back therein for pivotal movement of said impact dog about the cylinder axis of said recess between an inswung release position within the path of rotation of said tubular hammer and an outswung impact position intersecting said path of rotation, a pivot pin on said impact dog between the cylinder axis of the recess and the axis of rotation of said anvil, a cam in said recess flattened relative thereto and pivotally journalled on said pivot pin for normally occupying a central position in said recess, biasing means between said anvil on the one hand and said impact dog and flattened cam on the other for swinging in said impact dog to said release and said flattened cam to said central positions thereof, and cam means on said hammer responsive to relative rotation between said hammer and said anvil for urging said flattened cam to pivot along said recess whereby said impact dog is swung out to said impact position thereof.

14. A rotary impact motor according to claim 13 in which said impact dog is symmetrical with respect to a central plane therethrough and through said pivot pin, said central plane in the release position of said impact dog defining said central position of said flattened cam and substantially coinciding with a plane through the axis of rotation of said anvil and the cylinder axis of said recess.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,789,934 Dated February 5, 1974 Inventm-(s) Knut Christian SCHOEPS et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Page 1 in the heading, change Assignee to read as follows:

ATLAS COPCO AKTIEBOLAG-.

Signed and sealed this 30th day of July 197 (SEAL) Attest: v

McCOY M. GIBSON, JR. 0. MARSHALL DANN Attesting Officer Commissioner of Patents USCOMM-DC 60376-P69 u.s. covnunlurnlpmms orrlc: nu 0-:u-su.

ORM PO-IOSO (10-69) 

1. A rotary impact motor comprising a rotatable anvil, a hammer rotatably supported in coaxial relation with respect to said anvil, means for rotating said hammer, impact surface means on a portion of said hammer rotatable around said anvil, an impact dog pivotally journalled on said anvil radially inside the path of movement of said impact surface means and about an axis spaced from the axis of rotation of said anvil between an inswung release position within the path of rotation of said impact surface means and an outswung impact position intersecting said path of rotation, biasing means between said anvil and said impact dog for swinging in said impact dog to said release position thereof, and cam means between said hammer and said impact dog responsive to relative rotation therebetween for swinging out said impact dog from said release to said impact position into said path of rotation of said impact surface means.
 2. A rotary impact motor aCcording to claim 1 in which said cam means comprise a first cam means on said impact dog and cooperating second cam means mounted on said hammer for swinging out said impact dog to said impact position in same sense of swinging as the sense of rotation of said hammer.
 3. A rotary impact motor according to claim 1 in which said biasing means is a torsion spring extending axially of said impact motor, said torsion spring being non-rotatably associated with said anvil for swinging in said impact dog to said release position thereof.
 4. A rotary impact motor according to claim 3 in which said torsion spring is double shanked and provided with laterally protruding shank ends, opposed abutments on said anvil, a pin supported by said impact dog, and said opposed abutments and pin being resiliently straddled by said shank ends.
 5. A rotary impact motor according to claim 4, in which said cam means comprise a first cam means pivotally mounted on said pin of said impact dog and cooperating second cam means mounted on said hammer for swinging out said impact dog to said impact position thereof, a lug on said first cam means radially outwardly of said pin, and said lug being resiliently straddled by said shank ends.
 6. A rotary impact motor according to claim 1 in which said impact dog is elongated and has a partly cylindrical back, a partly cylindrical axial recess on said anvil member, and said recess being equiform with said back and slidably receiving said impact dog therein for pivotal movement relative to said anvil.
 7. A rotary impact motor according to claim 6 in which a radially directed face is provided on said impact dog opposite to said back thereof and concentric with the axis of rotation of said anvil when said impact dog is in the release position thereof.
 8. A rotary impact motor comprising a rotatable anvil, a hammer rotatably supported in coaxial relation with respect to said anvil, means for rotating said hammer bidirectionally, a pair of impact surfaces on peripherally spaced portions of said hammer facing oppositely in peripheral direction thereof and rotatable around said anvil, an elongated impact dog pivotally journalled on said anvil about an axis spaced from the axis of rotation of said anvil, said impact dog being pivoted between a release position with the opposed peripheral ends thereof within the path of rotation of said impact surfaces and an impact position with the impact dog pivoted in the same sense as the chosen sense of rotation of said hammer for swinging out one of said ends to intersect said path of rotation, biasing means between said anvil and said impact dog for returning said impact dog to said release position thereof, a first cam means on said impact dog, a second cam means mounted on said hammer and cooperating with said first cam means for pivoting said impact dog to the impact position thereof in response to relative rotation between said hammer and said impact dog, and means for mounting said second cam means on said hammer with a peripheral play corresponding to the peripheral spacing of said impact surfaces for causing said outswung end to be impacted by the one of said impact surfaces facing the chosen rotational direction.
 9. A rotary impact motor according to claim 8 in which said biasing means is a double shanked torsion spring provided with laterally protruding shank ends, opposed abutments on said anvil, a pin supported by said impact dog, and said opposed abutments and pin being resiliently straddled by said shank ends.
 10. A rotary impact motor according to claim 9, in which said cam means comprise a first cam pivotally mounted on said pin of said impact dog and a cooperating second cam mounted on said hammer for swinging out said impact dog to said impact position thereof, a lug on said first cam radially outwardly of said pin, and said lug being resiliently straddled by said shank ends.
 11. A rotary impact motor according to claim 1 in which said impact dog is symmetrical with respect to a central plane thErethrough, said central plane in the release position of said impact dog substantially coinciding with a plane through the axis of rotation of the anvil and the pivoting axis of said impact dog.
 12. A rotary impact motor according to claim 8 in which said impact dog is symmetrical with respect to a central plane therethrough, said central plane in the release position of said impact dog substantially coinciding with a plane through the axis of rotation of said anvil and the pivoting axis of said impact body.
 13. A rotary impact motor comprising a rotatable anvil, a tubular hammer supported in coaxial relation rotatably around said anvil, means for rotating said hammer, an elongated impact dog having a partly cylindrical back formed thereon, a partly cylindrical axial recess on said anvil member, said recess being equiform with said back and slidably receiving said back therein for pivotal movement of said impact dog about the cylinder axis of said recess between an inswung release position within the path of rotation of said tubular hammer and an outswung impact position intersecting said path of rotation, a pivot pin on said impact dog between the cylinder axis of the recess and the axis of rotation of said anvil, a cam in said recess flattened relative thereto and pivotally journalled on said pivot pin for normally occupying a central position in said recess, biasing means between said anvil on the one hand and said impact dog and flattened cam on the other for swinging in said impact dog to said release and said flattened cam to said central positions thereof, and cam means on said hammer responsive to relative rotation between said hammer and said anvil for urging said flattened cam to pivot along said recess whereby said impact dog is swung out to said impact position thereof.
 14. A rotary impact motor according to claim 13 in which said impact dog is symmetrical with respect to a central plane therethrough and through said pivot pin, said central plane in the release position of said impact dog defining said central position of said flattened cam and substantially coinciding with a plane through the axis of rotation of said anvil and the cylinder axis of said recess. 